1. Field of the Invention
The present invention relates to a fuel injection control device for an internal combustion engine and, more particularly, to a fuel injection control device for an internal combustion engine which is provided with a port injector to inject fuel into the intake port and an in-cylinder injector to inject fuel into the cylinder.
2. Background Art
As conventional internal combustion engines, those comprising a port injector to inject fuel into the intake port and an in-cylinder injector to inject fuel into the cylinder are known as disclosed in, for example, Japanese Patent Laid-open No. 2003-13784. In such a prior art internal combustion engine operating under certain conditions, port injection by a port injector is combined with in-cylinder injection by an in-cylinder injector so as to form a fuel-rich layer near the spark plug while introducing uniform mixture into the cylinder. According to this fuel injection technique, it is possible to keep lean the air-fuel ratio of the mixture while producing stable combustion in the cylinder. Hereinafter, such an internal combustion engine is denoted as a “dual-injector type internal combustion engine”.
In a dual-injector type internal combustion engine which concurrently performs both port injection and in-cylinder injection, the injection ratio between them must be controlled to an appropriate value. Conventionally, such an internal combustion engine therefore determines both the port injection fuel amount and the in-cylinder injection fuel amount at a predetermined injection amount calculation timing just before port injection is started. Then, the internal combustion engine successively drives the port injector and the in-cylinder injector so as to implement port and in-cylinder fuel injections of the determined respective amounts. According to this control technique, fuel can be injected into the intake port and the cylinder at an appropriate ratio, allowing stable combustion with a lean air-fuel mixture.
Including the above-mentioned document, the applicant is aware of the following documents as a related art of the present invention.
[Patent Document 1] Japanese Patent Laid-open No. 2003-13784
[Patent Document 2] Japanese Patent Laid-open No. 11-182283
[Patent Document 3] Japanese Patent Laid-open No. 5-231221
[Patent Document 4] Japanese Patent Laid-open No. 11-303669
In the above-mentioned prior art internal combustion engine, however, the port injection fuel amount and the cylinder injection fuel amount are calculated only once per engine cycle just before port injection is started. Therefore, if the load on the internal combustion engine changes or the change is detected after the calculation, the load change is not reflected in the fuel injection amount until the next engine cycle. More specifically, in the above-mentioned prior art internal combustion engine, any change in the load (intake air flow) during actual air intake, which may occur after the fuel injection amount is calculated and just before the port injection (intake stroke) is started, is not reflected in the fuel injection amount.
If the load change is not reflected in the fuel injection amount, no large change occurs in the torque of the internal combustion engine. This means that the conventional dual-injector type internal combustion engines leave room for improvement in terms of response to load changes.